1. Field of the Invention
The present invention relates to a phase feedback AFM (atomic force microscope) and to a control method for the phase feedback AFM. More particularly, the invention relates to a phase feedback AFM capable of maintaining constant the amplitude of the cantilever oscillation by controlling the amplitude of a driving voltage and to a control method for this phase feedback AFM.
2. Description of Related Art
A scanning probe microscope (SPM) is an instrument for imaging the surface of a sample by scanning a probe relative to the sample while the sample is placed close to the probe, producing a local stimulus from the probe to the sample during the scanning, and measuring a local response of the surface of the sample to the stimulus. Many types of SPM, which are different in method of measuring physical amounts, have been put into practical use. The family of scanning probe microscopy (SPM) includes scanning tunneling microscope (STM) for detecting an electrical current flowing between the probe and the sample, atomic force microscope (AFM) for detecting various forces exerted between the sample surface and the probe, and magnetic force microscope (MFM) for measuring the magnetic field distribution on the sample surface. In this way, numerous kinds of microscopes for measuring frictional force, viscosity, elasticity, and electric potential on the sample surface have been put into practical use.
An NC-AFM (non-contact atomic force microscope) is one kind of SPM, and is an instrument for imaging surface topography by detecting a shift in the resonance frequency of the cantilever due to the interaction between the probe and the sample and controlling the probe-sample distance to maintain the shift constant. Some methods are available to detect the resonance frequency of the cantilever. In the FM detection method, the cantilever is vibrated at its resonance frequency, and the resonance frequency is directly detected. In the slope detection method, the resonance frequency is indirectly detected as an amplitude or phase variation. In the slope detection method, amplitude variations are generally used. In contrast, in the present invention, an NC-AFM using phase variations is employed.
FIG. 2 is a diagram showing an example of configuration of a prior art instrument, i.e., an NC-AFM using the slope detection method. The rear surface of a cantilever 1 is irradiated with laser light emitted from a laser diode (LD) 2. The reflected light is detected by a photodetector 3. Vibrational displacements of the cantilever 1 are detected. The detector output is electrically amplified by a preamplifier 4 incorporating a bandpass filter.
Meanwhile, a driving signal is supplied from an oscillator 5 to a driving PZT (piezoelectric transducer) 6. The cantilever 1 is oscillated at the frequency of the driving signal supplied from the oscillator 5. The output signal from the photodetector 3 indicating displacements of the cantilever 1 is applied via the preamplifier 4 to an amplifier-detector 7, which outputs a voltage corresponding to the amplitude. For example, a lock-in amplifier or RMS-DC converter for converting an alternating signal into a direct current of an effective value can be used as the amplifier-detector 7.
The relationship between the gradient F of the force acting between the probe and a sample 17 and the resonance frequency f0 of the cantilever having a spring constant k is given byf0∝√{square root over (k−F)}The variation in f0 (frequency shift) substantially corresponds to the gradient F. At oscillations at a constant frequency close to the resonance frequency, a variation in the resonance frequency appears as a variation in amplitude.
A signal indicating the amplitude is applied to an error amplifier 8, which, in turn, controls the Z motion of a PZT scanner 11 via a filter 9 and a Z-piezo PZT driver 10 such that the amplitude becomes equal to a preset amplitude (i.e., the output from a reference signal source (Ref. V) 12). The amplitude that would otherwise vary is maintained at a constant value set by the reference signal source 12. The signal (i.e., the output signal from the filter 9) controlling the Z motion at this time represents the topography of the surface. The PZT scanner 11 is scanned in two dimensions by X and Y scan signals 13 X, Y. The Z motion produced at this time is used as a brightness signal. As a result, a topographic image is obtained.
In NC-AFM using such slope detection method, a phase signal may be displayed simultaneously as the variation of the resonance frequency of the cantilever 1. This phase signal is the phase difference between the reference signal from the oscillator 5 and the output signal from the preamplifier 4 indicating displacements of the cantilever 1. Variations in the resonance frequency of the cantilever 1 are more sensitively detected than variations in amplitude. The phase signal is produced by a phase detector 14. For example, a phase comparator is used as the phase detector 14. A highly sensitive phase signal may be used for feedback instead of an amplitude signal. In this case, any one of the output Vphase from the phase detector 14 and the output VAmp from the amplifier-detector 7 is selected by a switch SW and applied to the error amplifier 8.
In a known instrument of this kind, feedback control is used for scanning to control the distance between the probe and the sample surface such that the amplitude of oscillation of the probe is kept constant near the resonant frequency of the probe. The resilience is found from the amplitude of vibration of the oscillating probe varying based on modulation of the force acting between the sample and the probe. The viscosity is measured from the phase of the vibration of the probe (see, for example, JP-A-2000-346784 (paragraphs 0017-0021 and FIG. 2)).
In the prior art instrument, the amplitude varies simultaneously with the phase signal when the phase signal is used as a signal for controlling the distance between the probe and the sample. Consequently, there is the problem that the obtained topographic image deviates from the true surface topography by an amount corresponding to the variation in amplitude.